The main contacts of circuit breakers are often made of a material with a low electrical resistance, such as silver cadmium, to reduce the heat generated when the circuit breaker is closed. When the circuit breaker interrupts, an electrical arc is drawn between the separating contacts. The low resistance contact material, although beneficial for circuit breaker performance when the circuit breaker is closed, provides a poor surface for arcing since it erodes quickly under such circumstances.
In the prior art, an arc horn or arc runner was attached to the moving contact and/or stationary contact to provide a path for the arc to be blown off the respective contacts. For example, as shown in FIG. 1, a prior art circuit breaker 10 having an arc horn 16 is mounted adjacent the moving contact 12 and a lower runner 18 is positioned adjacent the stationary contact 14. Both the arc horn 16 and lower runner 18 are positioned between the contacts and the end of the circuit breaker. The arc that is drawn between the moving contact 12 and stationary contact 14 generates a high pressure which tends to force the arc out onto the arc horn 16 and lower runner 18. The end of the circuit breaker contains vents 26 which allow the arc to blow outwards from the high pressure area near the contacts to the low pressure area outside the circuit breaker 10.
Certain problems are associated with this prior art design. The vents 26 in the end of the circuit breaker 10 allow ionized gases to be blown towards the line terminal 20. This frequently results in phase to phase faults (i.e. current will flow from one conductor to another conductor) or phase to ground faults (i.e. current will flow from one conductor to ground) outside the circuit breaker in high voltage applications. The prior art design also increases the number of restrikes because the arc initially moves to the arc horn 16 and the lower runner 18 and is not blown into the arc stack 22 until later rather than early in the interruption process. As the voltage across the circuit breaker varies with the sinusoidal wave, the arc is free to move up or down the movable contact/arc horn path and the lower contact/lower runner path. This may result in a collapse of the arc or a restrike across the contacts after a current zero and will over time cause considerable erosion of the contact material.
The collapse of the arc is a particular concern because of the quantity of ionized gases generated in the immediate area of the arc. Because ionized gas has a lower dielectric recovery voltage than does air, a restrike is more likely to occur in the immediate area previously occupied by an arc.
The prior art design also fails to utilize the full dielectric capability of the arc stack 22. The arc is drawn from the arc horn 16 to the lower runner 18 and passes close to only a portion of the arc stack plates 24, for example, the lower 60% of the arc stack plates. The arc transfers to that 60% of the arc stack plates but does not utilize the dielectric recovery voltage available from drawing an arc between the upper 40% of the arc stack plates.